Immunochromatographic strip formats are increasingly popular for qualitative and semi-quantitative assays which use visual detection schemes. This type of immunoassay involves the application of a liquid test sample suspected of containing an analyte to be detected to an application zone of an immunochromatographic test strip. The strip is comprised of a matrix material through which the test fluid and analyte suspended or dissolved therein can flow by capillarity from the application zone to a detection zone where a visible signal, or absence of such, reveals the presence of the analyte. Typically, the strip will include means for immunospecifically binding the analyte to be detected with its specific binding partner which bears a detectable label. In one such scheme; as disclosed in U.S. Pat. No. 04,446,232; the strip contains an enzyme labeled, mobile binding partner for the analyte which is in a zone downstream from the sample application zone. If analyte is present in the test sample, it will combine with its labeled binding partner to form a complex which will flow along the strip to a detection zone which contains a substrate for the enzyme label capable of providing a colored response in the presence of the enzyme label. The strip contains a zone in which analyte is immobilized, so that labeled binding partner which does not combine with analyte, due to absence of analyte in the sample, will be captured and thereby inhibited from reaching the detection zone. There have been published various modifications of this technique, all of which involve some competitive specific binding system in which the presence or absence of analyte in the test sample is determined by the detection or lack thereof of labeled binding partner in the detection zone. In U.S. Pat. No. 4,868,108 there is disclosed a similar scheme with the addition of an immobilized capture reagent for the enzyme labeled binding partner in the detection zone to concentrate the enzyme label and enhance its ability to react with the enzyme substrate and thereby render the assay more sensitive.
Not all of the schemes for immunochromatography rely on an enzyme labeled binding partner/enzyme substrate as providing the signal for detection of the analyte. In U.S. Pat. No. 4,806,311 there is disclosed a multizone test device for the specific binding assay determination of an analyte and an immobilized binding partner therefore together with a detection zone for receiving labeled reagent which migrates thereto from the reagent zone. The detection zone contains an immobilized form of a binding substance for the labeled reagent. The labeled reagent bears a detectable chemical group having a detectable physical property which is detectable on the basis of its own physical properties, so that it does not require a chemical reaction with another substance. Exemplary of such groups are colored species fluorescers, phosphorescent molecules, radioisotopes and electroactive moieties.
U.S. Pat. No. 4,313,734 describes the use of gold sols as labels for antibodies which are detectable without a chemical change.
Immunochromatographic strip formats provide a viable system for the determination of various analytes (whether they be antigens or antibodies) but suffer from the limitation that they yield results which are at best semi-quantitative when, for some analytes, a quantitative answer is required. Accordingly, it would be desirable and it is an object of the present invention to provide a means for quantifying the results of analyses carried out by the use of immunochromatographic strip formats.